The present invention is directed to the fabrication of multilayer structures, and more particularly an improved physical-vapor deposition apparatus and method of use (and structure produced therefrom) for intra-layer modulated material deposition and assist beam.
It is well known in the prior art to utilize RF or DC magnetron sputter deposition systems for fabrication of thin film devices such as magnetic recording sensors and storage media. Such sputter deposition systems, commonly referred to as xe2x80x9cplasma sputtering deposition,xe2x80x9d are characterized by crossed electric and magnetic fields in an evacuated chamber into which an inert, ionizable gas, such as argon, is introduced. The gas is ionized by electrons accelerated by the electric field, which forms a plasma in proximity to a target structure. The crossed electric and magnetic fields confine the electrons in a zone between the target and substrate structures. The gas ions strike the target structure, causing ejection of atoms that are incident on a workpiece, typically a substrate on which it is desired to deposit one or more layers of selected target materials.
In the prior art conventional plasma sputtering deposition systems, relatively low operating pressures are utilized. This results in high translational energy atom and ion fluxes incident upon the substrate. This flux introduces manufacturing process difficulties, as device thicknesses become increasingly smaller. In particular, high levels of interfacial roughness and/or mixing are observed.
It is known in the prior art to utilize ion beam sputter deposition in certain applications to overcome some of the difficulties encountered with conventional RF/DC sputter techniques. Several aspects of ion beam sputter deposition, commonly referred to as xe2x80x9cion beam depositionxe2x80x9d (IBD), differ from conventional plasma sputter processes and provide significant advantages. For example, (1) the use of a lower background pressure results in less scattering of sputtered particles during the transit from the target to the substrate; (2) control of the ion beam directionality provides a variable angle of incidence of the beam at the target; (3) a nearly monoenergetic beam having a narrow energy distribution provides control of the sputter yield and deposition process as a function of ion energy and enables accurate beam focusing and scanning; (4) the ion beam is independent of target and substrate processes which allows changes in target and substrate materials and geometry while maintaining constant beam characteristics and allowing independent control of the beam energy and current density; (5) a second inert gas ion beam can be directed at the substrate to provide ion assisted deposition.
However, while the conventional IBD process has achieved much success, this conventional process also suffers from unacceptable high levels of interfacial roughness and interlayer mixing.
Also known in the prior art is to utilize molecular beam epitaxy (MBE) process to achieve physical-vapor deposition apparatus, as illustrated in U.S. Pat. No. 5,976,263 to Poole and U.S. Pat. No. 5,951,767 to Columbo the contents of which are incorporated herein by reference. In MBE, metal atoms are thermally evaporated and condensed onto a substrate. The atoms have low translational energies (xcx9ckT, where k is Boltzmann""s constant and T is the absolute temperature) of  less than 0.1 eV. During deposition, atomic assembly needed to form a high quality interface structure occurs by thermally activated diffusion on the grow surface. In conventional MBE, this thermally activated diffusion causes the grown films to suffer rough and interdiffused interfaces.
Several important applications, including giant magneto-resistive (GMR) exchange biased spin-valves thin-film read heads, photonic components, and semiconductor heterostructures, use multi-layer material stacks to perform various electronic, photonic signal processing and data storage functions. For instance, anti-reflection coating (ARC) films and dielectric optical filters utilize alternating layers of dielectric oxides with controlled thickness and roughness. Another application that uses multilayer material structures is the magnetic data storage industry. For instance, giant magneto-resistive (GMR) thin-film read head and magnetic random access memory (MRAM) concepts use multilayered material structures comprising stacks of non-magnetic conductive, ferromagnetic, and/or insulating material layers as thin as 10 to 30 xc3x85 (Angstrom).
In 1987, the giant magneto-resistive or GMR effect was discovered. GMR materials, usually consisting of at least two magnetic nanostructure entities separated by a nonmagnetic spacer. They display a large change of resistance upon the application of a magnetic field. GMR materials have a larger relative resistance change and have increased field sensitivity as compared against traditional anisotropic magneto-resistive or MR materials, such as Ni80Fe20 films. The improved relative resistance change and field sensitivity of GMR materials and related magnetic sensing elements allow the production of sensors having greater sensitivity and signal-to-noise ratio than conventional sensors. Thus, for instance, data storage systems using GMR read sensors can read data in smaller bit areas as compared to conventional read head devices. However, material stacks for fabricating GMR sensors generally use 6 to 8 layers of 4 to 6 different materials, as compared to the MR material stacks, which usually have only 3 layers of materials such as permalloy layers with Soft Adjacent Layers (SAL). Thus, creating material stacks for GMR read sensors generally requires more processing steps, including more complicated equipment and fabrication techniques for high-yield manufacturing of high-performance GMR thin-film heads.
In order to meet its goals for improved storage density, industry has turned to exchange biased spin-valve GMR thin-film read heads. Spin-valve GMR read heads are comprised of multi-layer depositions of 10 to 100 angstrom thick material films having precise thickness and microstructure control as well as extremely cohesive interface control at each interface of a multi-layer spin-valve GMR stack. Each spin-valve GMR stack must have good crystalinity in conjunction with abrupt and smooth material interfaces with minimal interface mixing to ensure proper GMR response and to establish excellent thermal stability. Essentially, GMR stacks may require controlled deposition of metallic multilayers which comprise ultrathin films as thin as about 5 to 10 atomic monolayers.
Another application for GMR materials is magnetic random access memories (xe2x80x9cMRAMxe2x80x9d), which are monolithic silicon-based nonvolatile memory devices presently based on a hysteretic effect in magneto-resistive or MR materials. MRAM devices are beginning to be used in aerospace and military applications due to their excellent nonvolatile memory bit retention and radiation hardness behavior. However, the MRAM devices can be easily integrated with silicon integrated circuits for embedded memory in a host of future applications in cell phones, personal computers, microprocessors, personal digital assistants (PDAs), etc. The implementation of GMR materials, such as spin-dependent tunnel junctions, could improve the electrical performance of MRAM devices to make MRAM devices competitive with semiconductor DRAM and flash EPROM memory devices. However, the performance of MRAM memory depends on precise control of layer thickness values and the microstructures of various thin films in a GMR stack of thin metallic films. Thickness fluctuations and other interface or microstructural variations in thin metallic layers can cause variation in MRAM device performance.
Similar difficulties can occur with periodic laminated multi-layer structures, such as laminated flux guide structures of iron, tantalum and silicon di-oxide.
As such, GMR materials have significant technological importance because they can be used to develop highly sensitive magnetic field sensors, read heads for disk drives, and MRAM that promise nonvolatility, radiation hardness, low power consumption, densities comparable to dynamic random access memory and access speeds comparable to static random access memory. All these applications require a high GMR ratio (defined as the maximum resistance change divided by the resistance at magnetic saturation), a low saturation magnetic field, a near-zero coercivity, a weak temperature dependence, and a high thermal stability. Many groups are now seeking to develop a vapor phase synthesis process that results in multilayers with this optimum combination of properties.
GMR properties are sensitive to nanoscale structural features of the films, their defect populations and the intrinsic properties of the material system. For instance, the lowest resistance appears to result from a sandwich structure with chemically separated planar interfaces. The GMR ratio therefore depends on nanoscale features of the multilayers such as the wavelength and amplitude of the interfacial roughness and the width and extent of interfacial chemical mixing. It may also be affected by grain texture, composition, layer purity, and the various types of lattice defects (including vacancies, voids, dis-locations, and twins) trapped in the films.
U.S. Pat. No. 5,661,449 to Araki et al. discloses forming a multilayer film of a plurality of magnetic and non-magentic layers alternatively stacked. The ""449 patent discloses forming the plurality of layers (104, 105, 106) with a deposition energy of 0.01 to 10.0 eV. However, the approach of the ""449 patent is unsatisfactory because it fails to account for the modulation required within each individual layer at the atomic monolayer application so as to provide for reduced interfacial roughness and layer intermixing as in the present invention.
FIGS. 1A and 1B illustrate the result of a conventional physical-vapor deposition process whereby the deposition energy is held constant during the deposition of each individual layer (104, 105, 106). A multilayer structure 100 having been deposited on a nickel substrate 101, having a growth direction in the y-coordinate direction. The orientation of the multilayer structure 100 and substrate 101 is defined by letting the x, y, and z coordinates correspond to the reference numbers 112, 111, and 110, respectively. The multilayer structure 100 is created by assigning atomic positions to an assembly of 960 atoms based on a fcc lattice with an equilibrium nickel lattice constant, a=3.5196 xc3x85. The substrate crystal consisted of 120 (224) planes in the x direction, 3 (111) planes in the y direction, and 16 (220) planes in the z direction. To prevent the crystal from shifting during adatom impact and minimize the effect of the bottom surface, the bottom two (111) planes were fixed. The multilayer structure 100 was deposited by alternatively depositing about 20 xc3x85 (approximately 10 monolayers) of copper (Cu) followed by about 20 xc3x85 (approximately 10 monolayers) of nickel (Ni).
Referring to FIG. 1A, copper and nickel atoms are marked by light and dark spheres, respectively. It can be seen that at the low incident energies (about 0.1 eV or less) and a fixed (normal) incident angle, xcex8=0 degrees, typical of either thermal evaporation (e.g., MBE) or high pressure (e.g., diode) sputtering, the interfaces 102 and 103 exhibit both significant interfacial roughness and copper layer intermixing in the subsequently deposited nickel layer.
Referring to FIG. 1B, when the multilayer structure 100 was deposited with an incident energy at 5.0 eV the interfacial roughness of both the copper on nickel interface 103 and the nickel on copper interface 102 were significantly reduced. However, the multilayer structure 100 suffers from excessive layer intermixing as the copper atoms are dispersed in the subsequently deposited nickel layer.
Turning to FIGS. 2A and 2B, other types of defects, including vacancies, twins, and dislocations are prevalent in conventional approaches. Typical examples of twin and dislocation structures are depicted in FIGS. 2A and 2B, respectively.
There is therefore a need in the art for an effective physical-vapor deposition process and system that produces a multilayer structure having reduced interfacial roughness and layer intermixing since these are critically important for spin-dependent electron transport.
The present invention has numerous applications including, but not limited thereto, for the growth of metal multilayers (e.g., magneto-electronic devices for sensing magnetic fields, magnetic random access memory, spin transistors and the like), semiconductor heterostructures including magnetic semiconductors, ceramic multilayers, optical filters or mirrors, x-ray mirrors, laser mirrors (with dielectric and metal multilayers), laser diodes, fiber optic waveguides and combinations of these material systems.
The present invention systems, devices, and structures will have a broad application including computers, peripheral computer components, cameras, telephones, televisions, miscellaneous electronic and communication components, and personal digital assistants (PDAs).
According to the present invention, a method of producing a multilayer structure by using a physical-vapor deposition apparatus is provided. In general the method comprises the steps of: forming a bottom layer having a first material wherein a first plurality of monolayers of the first material is deposited on an underlayer using a low incident adatom energy. Next, a second plurality of monolayers of the first material is deposited on top of the first plurality of monolayers of the first material using a high incident adatom energy. Thereafter, the method further includes forming a second layer having a second material wherein a first plurality of monolayers of the second material is deposited on the second plurality of monolayers of the first material using a low incident adatom energy. Next, a second plurality of monolayers of the second material is deposited on the first plurality of monolayers of the second material using a high incident adatom energy.
Accordingly, the incident energy can be ramped with the thickness of a given layer as the monolayers are accumulated/deposited. For example, the second monolayer has energy less than the third monolayer but more than the first monolayer, i.e., Enxe2x88x921 less than En less than En+1.
Some of the advantages of the present invention are that it provides an apparatus and method for fabricating multilayer structures that has reduced interfacial roughness and interlayer mixing.
These and other objects, along with advantages and features of the invention disclosed herein, will be made more apparent from the description, drawings, and claims that follow.